1. Field of the Invention
The present invention relates to an overvoltage-protective device for protecting a device such as a switching element against excessive voltage.
2. Description of the Prior Art
FIG. 1 is a circuit diagram showing a conventional overvoltage-protective device for a switching element S. This overvoltage-protective device, which is called a snubber circuit, is adapted to suppress abrupt rising voltage applied to the switching element S or absorb surge voltage. The switching element S formed by a power MOS or an IGBT (insulated gate bipolar transistor), for example, is adapted to switch current supplied to a high-voltage/high-current load such as a motor for PWM control, etc. Symbol A denotes a load circuit such as the motor, which serves as an overvoltage applying circuit applying excessive voltage to the switching element S for the reason that an unneglectable reactance component is present in a switch path of the switching element S or a motor or the like serves as a surge voltage source.
The overvoltage-protective device includes a diode 1, a capacitor 2 and a resistor 3 and is connected across terminals T1 and T2, which are drawn out across the switching element S. The capacitor 2 is adapted to absorb excessive voltage, and is charged at a high speed through function of the diode 1 which bypasses the resistor 3. The resistor 3 is adapted to discharge the capacitor 2 with an appropriate time constant.
When the switching element S is switched in a relatively low frequency range, the capacitor 2 functions as large resistance against a low frequency component, and hence value of current flowing through the capacitor 2 by charge/discharge thereof is relatively small. Discharge current through the resistor 3 is small in discharge, i.e., in an off state of the switching element S. Therefore, no disadvantage takes place to prevent a switching operation of the switching element S by, e.g., current continuously flowing to the load although the switching element S is turned off.
When surge voltage SG of high frequency component shown by a dotted line in FIG. 2 is applied to the switching element S, the capacitor 2 shows small resistance value with respect to a high frequency component, and hence the surge voltage SG is charged and absorbed by the capacitor 2 as overvoltage absorbing current I. A oblique line portion in FIG. 2 shows energy absorbed by the capacitor 2, and a solid line shows voltage actually applied to the switching element S. The charged capacitor 2 is slowly discharged through the resistor 3 after termination of the surge voltage SG, whereas influence by such discharge is small and not shown in FIG. 2.
Thus, the both ends of the switching element S are coupled in an AC manner by the diode 1 and the capacitor 2, to be shorted with respect to a high frequency component such as the surge voltage SG. A line S1 in FIG. 3 shows flowability of overvoltage absorbing current I responsive to a frequency of overvoltage such as surge voltage applied to the switching element S. Symbol SF denotes a frequency range in which the switching element S is switched. The overvoltage absorbing current I hardly flows in the switching frequency range SF so that the overvoltage-protective device exerts no influence on normal switching operation of the switching element S. A curve S2 shows absolute value of discharge current of the capacitor 2.
In the conventional overvoltage-protective device as hereinabove described, charge/discharge of the capacitor 2 is repeated through the diode 1 and the resistor 3 every time the switching element S is switched. Although the charge/discharge current is relatively small as hereinabove described, power loss accumulated in the diode 1 and the resistor 3 is unneglectably increased by such repetition. In order to effectively protect the switching element S against excessive voltage of a relatively low frequency, i.e., excessive voltage having relatively wide pulse width, it is necessary to increase capacitance value of capacitor 2 so that the overvoltage absorbing current I readily flows even if the frequency component is relatively low and amount of surge absorption is increased. In this case, however, a relatively large amount of the overvoltage absorbing current I flows in the switching frequency range SF shown in FIG. 6 of the switching element S, to prevent normal switching operation of the switching element S.
The capacitor 2 preferably have capacitance value as large as possible within a range not causing the aforementioned inconvenience, in order to improve the excessive voltage absorbing characteristic to the utmost. Further, the resistor 3 must be one for high power, in order to prevent destruction through heat generation caused by repeated charge/discharge of the capacitor 2. Such capacitor 2 and resistor 3 are prepared by discrete components since it is difficult to be formed as a monolithic IC, and hence the device cannot be reduced in size.